app.Use(async (ctx, next) =>
{
    await next();

    if(ctx.Response.StatusCode == 404 && !ctx.Response.HasStarted)
    {
        //Re-execute the request so the user gets the error page
        string originalPath = ctx.Request.Path.Value;
        ctx.Items["originalPath"] = originalPath;
        ctx.Request.Path = "/error/404";
        await next();
    }
});

app.UseRewriter(new RewriteOptions()                
   .AddRedirectToWww()
   .AddRedirect("^foo$", "bar")
   .AddRedirectToHttps()
);